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Name: Laura G.
Status: other
Age: old
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Does dew rise from the ground or does it fall?


Replies:
Laura, Dew neither rises nor falls. Dew is simply airborne water vapor that has condensed on cool objects. Since soil may contain more water than the air several feet above. As the grass cools off, nearby soil moisture condenses on it. Even so, wherever moist (humid) air has access to a cold surface, condensation will occur. It will "dew" it every time.

Regards,
ProfHoff


Dew or frost forms on objects that have cooled to less than the dew point or frost point (the point at which air is saturated with water), respectively.

Objects near the ground will cool more rapidly at night than the air itself, which is somewhat transparent to terrestrial radiation (heat energy leaving the ground). So dew or frost will normally occur before a fog will form.

The supply of water for dew or frost normally comes from water in the air, although excess water in the air can come from the soil and/or in plant leaves, particularly after a day of rain. Evaporation of water from the ground continues even at night and results in a shallow, more humid layer of air near the ground, resulting in some soaking dews on grass at times.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


Laura,

Since the source of the water is the aurrounding air, I would say it 'falls'. What actually happens is that the air containing a certain percentage of water vapor (expressed as a percentage of what it is able to hold at that temperature--namely, the air's relative humidity) cools and reaches a 'dew-point' temperature at which the relative humidity becomes 100%. This means that because cooler air can hold less water, the lower air temperature means that as the temperature falls further, the water vapor will condense from the air. Since the air closest to the ground, and the grass on it, is relatively cooler than the air above it, the condensation occurs there. As the temperature cools further, more and more water vapor will condense out. Note that in the morning if the sun begins to heat the surrounding air again, it can again hold relatively more water vapor and the dew will re-vaporize and be held in the air.

Thanks for using NEWTON!

Ric Rupnik


Dear Laura-

Dew forms when the air temperature cools enough to cause the humidity in the air to condense. The dew comes from the air, and condenses on objects on the ground. The moisture in the air may have originally come from the ground, but it evaporated into a gas, which later may condense as dew.

You can visit this web site for a more formal explanation of dew, the dew point temperature, and how dew forms.

http://lawr.ucdavis.edu/coopextn/frostprotection/fp003.htm

Wendell Bechtold


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